It is not too much to say that there is no time that we never see a light emitting device in our daily lives, such as a traffic light, a destination direction board at a station and an airport, a large size display disposed on the outer wall of a building, and further a backlight source in a cellular phone or the like. Thus, a light emitting device made of laminated semiconductors and a light receiving device to which the light emitting device is applied are indispensable, so that the need for improving the properties required for these devices continues increasing.
Particularly, a blue light emitting device has been developed behind the other three primary colors, namely, red, and green, and the blue light emitting device matching with the improvement of the property and each purpose is the most desired.
As such a blue light emitting device, a nitride semiconductor device including gallium (hereinafter, referred to as GaN system semiconductor device) is most used. As a structure of this GaN system semiconductor device, basically, a buffer layer composed of GaN on a sapphire substrate, an n-side contact layer composed of Si-doped GaN, an active layer containing an InGaN layer with a single quantum well structure or a multiple quantum well structure, a p-side cladding layer composed of Mg-doped AlGaN, and a p-side contact layer composed of Mg-doped GaN are stacked in this sequence. Further, on a surface of the n-side contact layer, on which a portion of the p-side cladding layer is etched to be exposed, an n-electrode composed of titanium/aluminum is formed, and on a surface of the remaining p-side contact layer, a p-electrode composed of nickel/gold is formed. This shows very excellent properties with a light emitting wavelength of 450 nm at 5 mW and an outside quantum efficiency of 9.1% at 20 mA.
FIG. 12 is a perspective view for showing an example of a conventional GaN system semiconductor device, and FIG. 13 is a top plan view thereof. In a GaN system semiconductor device having the above-described structure, on a substrate 301, an n-conductivity type semiconductor layer 302 including an n-side contact layer and an n-side cladding layer or the like, an active layer 303, and a p-conductivity type semiconductor layer 304 including a p-side cladding layer and a p-side contact layer or the like are layered in this sequence, and a p-electrode 306 is structured of laminated nickel/gold on a surface of the p-side contact layer. This p-electrode 306 is formed on nearly the entire surface of the p-side contact layer. An n-electrode 307 is disposed so as to be electrically connected to the n-conductivity type semiconductor layer 302.
Unless a p-type impurity is doped, the GaN system semiconductor represents the n-type. In order to-realize the GaN system semiconductor device having a p-n junction, the GaN system semiconductor representing the p-type is essential.
For example, subsequently to growth of GaN that is doped with Mg, further, using a method such as annealing and irradiation of an electron ray or the like, it is possible to obtain a p-type GaN. However, as it is obvious that GaN cannot be made into the p-type easily unless the particular method is applied, the GaN system semiconductor tends to be hardly made into the p-type, namely, the GaN system semiconductor representing the p-type tends to have a higher electric resistivity as compared to the GaN system semiconductor representing the n-type. In the case where the GaN system semiconductor layer has a high electric resistivity, an electric current applied to the light emitting device is hardly distributed in the p-type semiconductor layer, so that the emitted light due to a carrier recombination is biased with the light emission unevenly on the surface. As a countermeasure therefor, the p-electrode is formed on the entire surface of the p-side contact layer so that the electric current can flow evenly on the entire surface of the p-type semiconductor layer, resulting in minimizing the unevenness of the light emission.
In addition, nickel/gold can have translucency at 200 angstroms and good ohmic contact with the p-type GaN semiconductor device representing the p-type, so that it is preferably used as a p-electrode material.
However, since gold has the nature of absorbing light having a wavelength shorter than about 550 nm, when gold is used as a p-electrode material, it may absorb the light emitted under the p-electrode. As a result, the light emitted inside the device is not effectively emitted outside.